Hey Andy -
Not sure what a "linear" .ies file is, but .ies is one of my requirements.
I'd also like to see this kind of tabular comparison across a lot of features. I notice that several renderers have .ies compatibility, but only once upgraded to the "pro" versions.
I also like Twilight's material editor (at least, as much as I've experimented with it in the demo) but then I see things like this texture for Maxwell http://resources.maxwellrender.com/news_scripts.php?t=1#page=2&mode=1&id=5540 and wonder what other renderers can do "that".
Another biggie is subsurface scattering - or, at least, proper handling of light through transparent and translucent materials. For example, a "Japanese screen" that properly transmits illumination when being hit on one side with either bright ambient illumination, sunlight through trees (shadows), or a small bright spotlight. I don't know how to look this feature up comparitively.
[edit] perhaps a good way to create a tabular comparison is to create the categories and ask the companies who make the renderers to fill in the details. Not sure what the best way to create the categories would be, but I assume a group effort may actually pay off in this case...
cheers,
Andrew